Photographic processes using silver halides have been most widely used because of their superior photographic characteristics such as sensitivity and the adjustment of gradation to other photographic processes such as an electrophotographic process and a diazo photographic process. In recent years, techniques capable of easily and rapidly obtaining images have been developed by changing the method of image formation and processing of silver halide photographic materials from the conventional wet treatment with developers or the like to a dry treatment by heating or the like.
Heat-developable light sensitive materials are known in the art. Heat-developable light sensitive materials and heat developing processes are described, for example, in Fundamentals of Photographic Engineering, pages 553-555, published by Corona Company, 1979; Image Information, page 40, published in April 1978; Nebletts Handbook of Photography and Reprography, 7th edition (Van Nostrand Reinhold Company), pages 32-33; U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, and 3,457,075; British Pat. Nos. 1,131,108 and 1,167,777; and Research Disclosure, June 1978, pages 9-15 (RD-17029).
Many methods have already been proposed for obtaining colored images by a dry process. With regard to the method of forming a colored image by the combination of an oxidation product of a developing agent with a coupler, U.S. Pat. No. 3,531,286 proposes a p-phenylenediamine reducing agent and a phenolic or active methylene coupler; U.S. Pat. No. 3,761,270, a p-aminophenolic reducing agent; Belgian Patent No. 802,519 and Research Disclosure, September 1975, pages 31-32, a sulfonamidephenol-type reducing agent; and U.S. Pat. No. 4,021,240, a combination of a sulfonamidephenol-type reducing agent and a 4-equivalents coupler.
These methods, however, have the defect that since an image of reduced silver and a color image occur simultaneously in the exposed portion after heat development, the color image becomes hazy. This defect may be eliminated by removing the silver image by a wet process, or transferring only the dye to a sheet having another layer such as an image receiving layer. It is not easy, however, to distinguish the dye from the unreacted material and transfer only the dye to the other layer.
Research Disclosure, May 1978, pages 54-58 (RD-16966) describes a method in which a silver salt is formed by introducing a nitrogen-containing heterocyclic group into the dye, and liberating the dye by heat development. According to this method, it is difficult to inhibit the liberation of the dye at portions not exposed to light. Hence, a clear image cannot be obtained, and the method is not applied generally.
With respect to a method of forming a positive color image by a thermosensitive silver dye bleaching process, useful dyes and a method of bleaching are described, for example, in Research Disclosure, April 1976, pages 30-32 (RD-14433), ibid. December 1976, pages 14-15 (RD-15227), and U.S. Pat. No. 4,235,957. In this process, an activator sheet must be superimposed during heating in order to quicken the bleaching of the dye, and an extra step and an extra material are required. It further has the defect that since the resulting color image is gradually reduced and bleached by free silver, etc., present together, it cannot withstand storage for an extended period of time.
A method of forming a color image by utilizing leuco dyes is described, for example, in U.S. Pat. Nos. 3,985,565 and 4,022,617. This method, however, has the defect that a leuco dye is difficult to include stably in a photographic material, and during storage, the photographic material is gradually colored.
Photographic materials capable of eliminating the defects of these conventional methods, and a method for image formation on such photographic materials are described in Japanese Patent Application (OPI) No. 149046/1983 (the term "OPI" as used herein mean a "published unexamined Japanese Patent Application"), and Japanese Patent Application No. 42092/1983 (corresponding to U.S. patent application Ser. No. 590,592 field on Mar. 16, 1984). In these methods, it is necessary to move a mobile dye formed imagewise to a dye fixing layer. For this purpose, a solvent for movement of the dye, such as water, a basic aqueous solution, an alcohol or dimethylacetamide is supplied, or a hydrophilic thermal solvent is included in the photographic material in advance so that the supply of solvent from outside is not particularly required. The latter method is advantageous because of its simplicity.
Furthermore, in the above image-forming methods, the combined use of a dye releasing activator such as a base is convenient in order to form a color image of high density rapidly.
However, many of these additives are chemically active, and when they are present together with silver halides, they may impair photographic characteristics or change storage stability. Furthermore, since the hydrophilic heat solvent is used in large quantities, its inclusion in the light sensitive material necessarily results in an increase film thickness. Consequently, the quality of the film is degraded or the sensitivity of the light sensitive material is reduced. Hence, the sharpness of the image is reduced.
Japanese Patent Application (OPI) No. 18351/1984 discloses a dye fixing material for eliminating such defects. This color fixing material does not have sufficient color fastness after storage in either dark or light room, and there is still room for improvement in peeling property between the image forming material and the dye fixing material after heat transfer.